The present invention relates to a position measuring instrument comprising a turning unit and a fixed unit, in which a rotating unit has a reflection member and a slant measuring unit can detect a backlash of the turning unit to correct the backlash.
Conventionally, an automatic tracking total station which is made by automating a total station is known in the instrument that automatically measures a position of a target to be measured. The automatic tracking total station is so devised that a direction of collimation of a lens barrel can rotate up and down, and right and left, by driving force of a motor. Rotation angles of horizontal rotation and vertical rotation are detected using the output of an encoder as an angle measuring means.
Not only a collimation telescope and a distance measuring means, which are included in an ordinary total station, but also a tracking means for detecting a prism reflector placed on a measurement position are built into the lens barrel.
Besides them, a main body of the automatic tracking total station is provided with a tilt sensor for detecting inclination of a frame, various kinds of electronic processing circuits, an indicator, a battery and the like.
FIG. 5 is a partial sectional view of the automatic tracking total station.
A surveying-instrument main body 1000 comprises the following: a lens barrel 1100 that is provided in such a manner as to rotate vertically; a frame 1200 that supports the lens barrel 1100 so that it can rotate vertically; a base 1300 that supports the frame 1200 so that it can rotate horizontally; and a leveling unit 1400 having a leveling function, which is placed under the base 1300.
The frame 1200 is provided with a horizontal rotation mechanism 1710 that rotates the frame 1200 horizontally about a vertical axis through a horizontal rotation drive gear 1711 using a horizontal rotation motor 1712, and a vertical rotation mechanism 1740 that rotates the lens barrel 1100 vertically about a horizontal axis through a vertical rotation drive gear 1741 using a vertical rotation motor 1742.
A horizontal rotation-angle detecting encoder 1730 is attached to a horizontal rotation axis 1720, and an angular-height detecting encoder 1760 is attached to a vertical rotation axis 1750, so that they detect respective rotation angles.
The automatic tracking total station is basically made by remodeling a surveying instrument of the manually operated total station type so that the surveying instrument can be driven by a motor. An automatic tracking device is a device which is automated so that the device requires no operator. However, a collimation telescope, a distance measuring means, a tracking means, and the like, are built into a lens barrel.
Therefore, the automatic tracking device becomes large and heavy. In a similar manner, not only the lens barrel but also a drive unit for vertical and horizontal rotation are built into the frame that supports the lens barrel and that rotates horizontally. This makes a turning unit large and heavy.
For reasons of its use, the surveying instrument is so devised that the lens barrel rotates in a vertical direction, and that the frame rotates in a horizontal direction with a telescope unit.
The torque required when accelerating a rotating unit from a stopped state can be expressed in an equation below.T=I*dω/dt  First Equationwhere T is torque, I is moment of inertia of a rotation system, and dω/dt is angular acceleration.
This first equation shows that the torque is proportional to the angular acceleration and the moment of inertia.
Since the surveying instrument is a measuring instrument that is generally used outdoors, a battery is usually used as its power supply. Therefore, power consumption of a motor is required to be minimized. This means that it is necessary to use a small-size motor for this purpose. As a result, the torque T which can be generated by the small-size motor is naturally limited.
On the other hand, as for the automatic tracking total station, it is necessary to build a motor into a part corresponding to vertical and horizontal rotation axes to rotate a telescope unit and a frame so that the telescope is quickly directed toward a direction of the prism which is a target to be measured. Further, if the automatic tracking total station measures a prism as a moving target to be measured while tracking the prism, an improvement in follow-up properties is particularly required.
In both of the cases, that is to say, increasing the speed of rotation, and increasing the follow-up properties., performance in acceleration and deceleration of a rotating unit becomes important in particular. When considering how to improve the performance in acceleration and deceleration, using a large motor to improve the torque T is a simple and easy method. However, as described above, this method causes an increase in power consumption and an increase in weight of the motor. After all, this method is not a wise policy.
As shown in the first equation, if the torque T is fixed, and if an increase in angular acceleration is required, it is necessary to reduce the moment of inertia I. The moment of inertia I is proportional to mass of the rotating unit, and is proportional to the square of a turning radius.
Accordingly, if a decrease in moment of inertia I is required, decreasing the mass and turning radius of the rotating unit suffices. This means that even if the torque T is small, performance in rotation can be improved.
As described above, the conventional surveying instrument is so devised that the whole lens barrel or the whole frame rotates. As a result, many component parts are built into these parts. Therefore, the structure of the conventional surveying instrument poses the problem that it is difficult to improve the performance in acceleration and deceleration.